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Trammell, JH, Jiang X, Li LM, Kao A, Zhang GJ, Chang EKM, Yung Y.  2016.  Temporal and spatial variability of precipitation from observations and models*. Journal of Climate. 29:2543-2555.   10.1175/jcli-d-15-0325.1   AbstractWebsite

Principal component analysis (PCA) is utilized to explore the temporal and spatial variability of precipitation from GPCP and a CAM5 simulation from 1979 to 2010. In the tropical region, the interannual variability of tropical precipitation is characterized by two dominant modes (El Nino and El Nino Modoki). The first and second modes of tropical GPCP precipitation capture 31.9% and 15.6% of the total variance, respectively. The first mode has positive precipitation anomalies over the western Pacific and negative precipitation anomalies over the central and eastern Pacific. The second mode has positive precipitation anomalies over the central Pacific and negative precipitation anomalies over the western and eastern Pacific. Similar variations are seen in the first two modes of tropical precipitation from a CAM5 simulation, although the magnitudes are slightly weaker than in the observations. Over the Northern Hemisphere (NH) high latitudes, the first mode, capturing 8.3% of the total variance of NH GPCP precipitation, is related to the northern annular mode (NAM). During the positive phase of NAM, there are negative precipitation anomalies over the Arctic and positive precipitation anomalies over the midlatitudes. Over the Southern Hemisphere (SH) high latitudes, the first mode, capturing 13.2% of the total variance of SH GPCP precipitation, is related to the southern annular mode (SAM). During the positive phase of the SAM, there are negative precipitation anomalies over the Antarctic and positive precipitation anomalies over the midlatitudes. The CAM5 precipitation simulation demonstrates similar results to those of the observations. However, they do not capture both the high precipitation anomalies over the northern Pacific Ocean or the position of the positive precipitation anomalies in the SH.

Zhang, GJ, Wang HJ.  2006.  Toward mitigating the double ITCZ problem in NCAR CCSM3. Geophysical Research Letters. 33   10.1029/2005gl025229   AbstractWebsite

The appearance of a spurious Inter-Tropical Convergence Zone south of the equator in the eastern and central equatorial Pacific, in addition to the observed one north of the equator, is a common problem in coupled global climate models. The present study investigates this "double ITCZ'' problem in the NCAR CCSM3. It shows that use of a modified Zhang-McFarlane convection scheme significantly mitigates the double ITCZ problem in boreal summer. This has a profound impact on the simulated sea surface temperature through cloud radiative forcing feedback. Both the warm bias in the southern ITCZ region and the cold bias in the cold tongue over the equator are reduced. Examination of the time series of precipitation, SST and surface energy fluxes shows that, depending on the convection parameterization used, double or single ITCZ emerges quickly within the first few months after the model start.